1. Field of the Invention
The present invention relates to an apparatus and a method for preparing a single crystal made of silicon in accordance with the Czochralski method. An essential feature of this method is that a monocrystalline seed crystal in a receiver sealed off from the environment is brought into contact with molten material provided in a crucible and is pulled away from the melt at a certain rate, a single crystal starting to grow in the process at the underside of the seed crystal.
2. The Prior Art
Many pulling apparatuses are equipped with a tubular to conical body which shields the growing single crystal. The U.S. Pat. No. 4,330,362 describes a body which shields the single crystal and tapers conically toward the melt, having a cup-like appearance as a result. Such bodies which concentrically enclose the growing single crystal over its entire length or part thereof screen off heat radiation emanating from the crucible wall. One consequence of this is that the single crystal can be pulled at a higher rate.
While a single crystal is being pulled, silicon monoxide is being formed continuously, which escapes as a gas into the receiver chamber above the melt. The receiver is purged with inert gas to prevent the silicon monoxide from being deposited as a solid on cold parts in the crucible region of the pulling apparatus, from reaching the crystallization front at the single crystal and from causing dislocations in the crystal. Customarily, inert gas is conducted toward the surface of the melt and, together with silicon monoxide formed, is exhausted from the receiver. A body shielding the single crystal divides the receiver chamber above the melt into an inner and an outer portion. An inert gas stream for removing silicon monoxide is conducted, as a rule, through the inner portion of the receiver chamber toward the surface of the melt. It then passes, between the lower end of the shielding body and the surface of the melt, into the outer portion of the receiver chamber and finally to an exhaust port provided in the receiver wall.
When crystals doped with antimony or arsenic are prepared, the striking fact emerges that certain product specifications requiring a high dopant content in the single crystal are virtually unattainable or are attainable only with a very low yield. Incorporation of dopant in high concentrations becomes the more difficult, the larger the diameter of the growing single crystal. The vapor pressure of these dopants in liquid silicon is fairly high, so that the melt is rapidly depleted of dopant. Attempts have therefore been made to supersaturate the melt with dopant in order to circumvent a dopant deficiency. Unfortunately, such measures promote the formation of dislocations in the single crystal, which then has to be melted back in a time-consuming procedure, before pulling can continue. In the meantime, as a rule, so much dopant will have left the crucible that the demanded rate at which the dopant is to be incorporated into the crystal lattice can no longer be achieved.